An element, such as GaAs semiconductor circuit chip, is housed in an air tight cavity in an assembly package to ensure that the chip functions in the environment in which it is designed to operate. Typically, the assembly package comprises a set of conductive frame leads and a frame pad, a semiconductor chip mounted on the pad, and a shell housing the chip. The chip is bonded to the frame pad and wire bonded to the leads of the lead frames. The cavity is formed by the shell covering the chip and the frame pad. The pad is backed by a dielectric backer plate or molded plastic.
According to a known method of making a cavity for the assembly package, a layer of epoxy in A-stage is attached to the lip of the shell and to the backer plate. Upon heating, the A-stage epoxy layer melts into a gel and bonds the backer plate to the bottom of the frame pad and the shell lip to the top of the base and the frame leads. The package is heated to a first temperature for a given duration to harden the epoxy.
In curing the epoxy, care is taken to raise the temperature outside the cavity to a temperature equal to that inside the cavity, while the epoxy is still in a liquid state. Care is taken to time the placing and pressing of the shell lip against the epoxy and covering the assembly in an enclosure to raise the outside temperature to an isothermal state while curing the epoxy. Later the assembly is placed in an enclosed oven at yet an other elevated temperature for a predetermined duration to cure the hardened epoxy.
It has been found that a known methods are difficult apply provide low production yield and poor quality products. A high percentage of the product made by the known method has been found to fail the industry standard condition "c" gross leak test and accelerated life cycle tests involving heating and freezing cycles, due to the poor quality seal of the cavities due to the fissures and cracks.
Despite the teachings of the prior art, a need still exists for a method of making an assembly package having an air tight cavity for housing an element therein which yields a high percentage of quality products which pass the industry's standard condition "c" gross leak test and accelerated life cycle tests, in an efficient manner and at an economical manufacturing cost.